As demand for ever-shrinking semiconductor devices continues to increase, so too will the demand for improved lithographic processing techniques used for fabricating semiconductor devices. Lithographic printing tools, such as scanners and steppers are required to maintain focus within the range of ±10 nm around the nominal focus position for recent node and this requirement will become even more stringent in the future.
Prior methods to enhance pattern sensitivity to focus changes include the use of isolated lines. Further, software is used to generate a library of simulated spectra, using material properties, such as n and k values, of the resist, antireflective coatings, planarization films and the like. Such approaches further include the use of nominal test target structure parameters (e.g., line width, sidewall angle and height), reasonable expected ranges of target structure parameter variation, and scatterometry tool optical parameters (e.g., wavelength range, azimuth angle range, angle of incidence range, polarizations and etc.). Spectra are then collected from test targets for all combinations (if possible) of focus and exposure in a focus-expose matrix (FEM) on a test wafer and stored in library. Then, during measurement a closest match is found is the library for each spectrum and the corresponding structure parameters (e.g., line width, sidewall angle, line height and etc.) for the library matches are matched with the programmed focus and exposure combinations from the FEM.
In additional settings a line end shortening effect is measured utilizing an imaging tool. This approach suffers from low sensitivity, which is compounded near the best scanner focus position, where the behavior of the measured target parameter is parabolic with respect to scanner focus change.
The utilization of phase shift masks allows for the achievement of high sensitivity to scanner focus changes by converting changes in focus to alignment errors, which that can be measured with an overlay measurement tool. However, phase shift masks are typically not used in production as they lead to significant cost increases. As such, it is desirable to provide improved methods and systems that act to cure the defects of the prior art.